Physiology of psychogenic movement disorders

Abstract

Psychogenic movement disorders (PMDs) are common, but their physiology is largely unknown. In most situations, the movement is involuntary, but in a minority, when the disorder is malingering or factitious, the patient is lying and the movement is voluntary. Physiologically, we cannot tell the difference between voluntary and involuntary. The Bereitschaftspotential (BP) is indicative of certain brain mechanisms for generating movement, and is seen with ordinarily voluntary movements, but by itself does not indicate that a movement is voluntary. There are good clinical neurophysiological methods available to determine whether myoclonus or tremor is a PMD. For example, psychogenic myoclonus generally has a BP, and psychogenic stimulus-sensitive myoclonus has a variable latency with times similar to normal reaction times. Psychogenic tremor will have variable frequency over time, be synchronous in the two arms, and might well be entrained with voluntary rhythmic movements. These facts suggest that PMDs share voluntary mechanisms for movement production. There are no definitive tests to differentiate psychogenic dystonia from organic dystonia, although one has been recently reported. Similar physiological abnormalities are seen in both groups. The question arises as to how a movement can be produced with voluntary mechanisms, but not be considered voluntary.

Fig. 1

The movement-related cortical potential showing a normal-looking Bereitschaftspotential associated with psychogenic abdominal jerks. The trace is an average of 50 trials with zero time being the electromyography (EMG) onset of activity in rectus abdominus. Note the electroencephalography negativity at electrode Cz well before the EMG onset. In clinical practice the signal-to-noise ratio is often not as good as those commonly illustrated with hand movements because the movements may be unusual, like those here, and it may not be possible to get as many traces as desired.(Record obtained from an National Institutes of Health patient by Dr Codrin Lungu).

Fig. 2

Measurements taken from accelerometric recordings from the right and left wrists showing entrainment with voluntary tapping. The results here are from a difficult case and show some of the subtleties of these observations. The patient's primary abnormality was a tremor of the right upper extremity. The left panels are recordings taken with both wrists extended. Tremor power is measured in (mg)², which is one-thousandth of a gravity unit, squared. There is a tremor with peak frequency of 7.5 Hz in the right wrist. On the left side, there is a very low amplitude tremor at 6.7 Hz (note the scale of the power axis) that likely represents a physiological tremor. On the right-hand panels are the recordings of when the patient was asked to tap with the left hand at 3.0 Hz, matching a metronome. This is done well at 3.05 Hz and there is a prominent harmonic at 6.10 Hz. Looking at the raw accelerometric trace in the inset, the harmonic comes from a “notch” on the main peaks, apparently from a bounce as the patient was tapping on the table. The important observation is of the acceleration of the right wrist that shows a tremor frequency now at 6.1 Hz, with a sub-harmonic at 3.05 Hz. The change from 7.5 Hz to 6.1 Hz demonstrates the entrainment. The 6.1 Hz frequency, as seen in the raw traces, is a sinusoidal rhythm at twice the left-hand tapping rate. One tap is synchronous with the left hand, and the subsequent tap falls in the middle of the interval to the next tap, which is also synchronous with a left-hand tap. (Record obtained from a National Institutes of Health patient by Dr Dietrich Haubenberger).

Fig. 3

Multiple traces from accelerometer on the right wrist of the same patient as in Figure 2. The recordings show a brief pause in the psychogenic tremor with a ballistic movement of the other hand. The traces are aligned at 0 when there was the auditory cue for a ballistic movement of the left wrist, reaction time was about 200 ms, and the traces extend for 2 s before and after the cue. The pause in the right wrist tremor is clearly seen on every trace. (Record obtained from a National Institutes of Health patient by Dr Dietrich Haubenberger).